The HBO Statement continues, "California Horse Racing Board (CHRB) official veterinarian Dr. Gary Beck stated, 'I had just examined the horse as part of our routine health and safety procedures prior to work that would be done later on the track. The horse was on her way back to the stall when she reared, flipped over backwards, and struck her head on the ground. Fortunately, attending veterinarian Dr. Heidi Agnic was there to administer immediate aid to the injured horse and determined that humane euthanasia was appropriate.'

"CHRB Equine Medical Director Dr. Rick Arthur said, 'As with all fatalities within CHRB racing enclosures a necropsy will be conducted. Unfortunately, we see several of these injuries in the stable area every year. They are more common than people realize.'

We're starting to realize.

"An American Humane Association Certified Safety Representative was on the premises when the accident occurred, and all safety precautions were in place," adds the network's statement. "HBO and everyone involved with the production are deeply saddened, and are working in full cooperation with the AHA and the CHRB to complete their inquiry."

A spokesperson for PETA expressed concerns with the ongoing tragedies, telling E! News after the first two deaths, "Horses don't break down without a reason of breaking down. And so we want to know…we want to know, first of all, who were the horses. They were allegedly former race horses; we want to know their names.

"We can look at their racing records, we'd like to see the necropsy reports, because we'd like to know what was found. We have some serious concerns with the way that they were filming by running the horses and then resting them and running them again. Because you know, for a horse, a horse doesn't know he's acting. For a horse, it's I'm running, I'm stopping, I'm running, I'm stopping. And even if it's done only three times, which doesn't seem like very much to us, that's a lot for a thoroughbred.

"That's a lot of running in a single day for a horse who is a trained racing horse. They just don't do that. And so we have some really serious concerns about that. But we haven't received any more information about it yet."

UPDATE: An HBO rep tells E! News, "We will not be filming horses until American Humane Association completes its inquiry. Production will continue with other scenes."

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